Activation volumes were extracted from the effect of pressure on the ionic conductivities of CaF2, SrF2, BaF2, and CdF2. The activation volumes for motion of vacancies were measured in oxygen-contaminated samples and used to calculate defect-formation volumes from the activation volumes at high temperatures. The formation volumes are uniformly small and consistent with the dominance of Frenkel defects for temperatures up to 1400 K in the alkaline-earth fluorides. The CdF2 was found to contaminate badly at temperatures around 600 K, and become an electronic conductor. All of the crystals were found to exhibit frequency-dependent conductivities at high temperatures, attributable to electrode effects or bulk dielectric response of the material, or both. © 1980 The American Physical Society.